This application is related to acoustic inkjet printing and more particularly to an acoustic inkjet print head with individual control circuits for the piezo-electric transducer of each aperture to provide an acoustic wave with single optimized frequency.
Referring to FIG. 1, there is shown a portion of a prior art acoustic inkjet print head 10. Print head 10 has a housing 12, which contains a sheet of glass substrate 14 and ink 16 over the glass substrate 14. Housing 12, has a plurality of apertures 18, each of which is dedicated to a pixel. Under the glass substrate, there is a plurality of piezo-electric transducers 20. For the purpose of simplicity, hereinafter, the "piezo-electric transducer" is referred to as "transducer". Each transducer 20 is dedicated to one aperture 18 and is located directly across its respective aperture 18. Once each transducer 20 is activated, it will oscillate and generate acoustic waves 22. The acoustic waves 22 travel within the glass substrate 14 toward the ink 16.
Over the glass substrate 14, there is a plurality of Fresnel lenses 24, each of which corresponds to one of the transducers 20 and is located across from its respective transducer 20. The Fresnel lenses 24 receive the acoustic waves 22 from the transducers 20 and focus the acoustic waves onto their respective aperture 18. The focused waves 22 cause the ink to be ejected from the apertures.
Transducers 20 receive an RF frequency signal from an oscillator 30. Oscillator 30 generates an RF signal and sends it to an RF Amplifier 32 to be amplified. The amplified RF signal is sent to several RF power splitters 34. Each output of power splitters 34 is distributed between the plurality of transducers 20.
The output of each power splitter 34 is connected to a set of transducers 20 through individual switches S.sub.1 for providing RF signal to respective transducers 20. Switches Sare controlled by pixel information. Based on the pixel information, when a given pixel needs ink, switch S.sub.1 of a respective transducer closes to send the RF signal to that transducer for activating the transducer and causing ink to be ejected from the respective aperture 18.
In operation, the acoustic waves 22, which are focused onto the apertures 18 will partially be reflected by the surface 50. The reflected waves interfere with the original waves 22. If the impedance of either the ink 16 or Fresnel lens 24 does not match that of the glass substrate 14, the resulting stack of glass substrate 14, Fresnel lens 24, and the ink 16 will operate as a cavity. Hereinafter the combination of cavity (glass substrate 14, Fresnel lens 24 and the ink 14) and transducer will be referred to as resonant stack. Depending on the frequency of the original waves and the cavity length, the reflected waves can have a different phase than the phase of the original waves.
Referring to FIG. 2, there is shown the resonance distribution of a resonant stack. FIG. 2, shows the effect of the interference between the original acoustic waves and the reflecting acoustic waves. Referring to both FIGS. 1 and 2, if the reflected waves in the glass substrate 14 have opposite phase as that of original waves, then they will cancel the original waves 22 (cancellation C). However, if the reflected waves have the same phase as that of the original waves 22, they will increase the amplitude of the original waves (spikes S). Any phase between the two extremes of in-phase or the opposite-phase will interfere constructively or destructively with the original waves depending on if the phase is closer to in-phase or to the opposite phase respectively.
In this approach, an external frequency from the oscillator 30 is applied to each transducer 20 to cause the transducer to oscillate. Typically in the absence of an external frequency, if each transducer 20 starts oscillating, it will oscillate at a resonance frequency which is defined by the resonant stack. Usually, the external frequency does not match the resonance frequency of the resonant stack and as a result, the transducers 20 generate acoustic waves which do not resonate with the resonant stack. In addition, manufacturing tolerances cause each resonant stack to oscillate at a unique frequency.
Since the transducers oscillate at different frequencies than the resonance frequencies of the resonant stack, spikes or cancellation can occur. As can be observed, the spikes S occupy a small percentage such as 5% of the distribution and the majority of distribution is cancellation. This reduces the efficiency of the transducers. In addition, depending on the acoustic waves generated by the transducers 20, the intensity of the acoustic waves will vary strongly.
This problem is usually resolved in two ways. One approach is to deposit a matching layer over the glass substrate 14. This layer compensates for the mismatched impedance of the ink 16, the Fresnel lenses 24 and the glass substrate 14 and causes a reduction of amplitude of the reflected waves. Therefore, the reflected waves do not interfere as strongly with the original waves.
Another approach is to sweep or chirp the RF frequency to vary the frequency of the transducer's oscillation in order to generate acoustic waves with variable frequencies. Varying the frequency within a range gradually from one end of the range to the other end of the range is called "sweeping" or "chirping". By chirping the frequency, the resulting waves will have the effect of the average of all the waves with different frequencies and therefore average out the resonance spike problem.
This configuration has several problems. The inefficiency of the transducers causes the control circuit 36 to dissipates a great amount of RF energy. The control circuit 36 has to be fully on regardless of the number of active transducers. In addition, the external frequency applied to the transducers has to be chirped, which in turn causes the acoustic waves generated at the transducer to have a varying frequency. With a varying frequency, at any given time, the reflected waves will have a different phase. Therefore, due to the varying phase of the reflected waves, the waves reaching each aperture will have an average frequency and amplitude.
It is an object of this invention to eliminate the high power dissipation. Also, it is another object of this invention to individually control and adjust each transducer to maximize the intensity of the acoustic waves when they reach their respective apertures and reduce the net amount of RF power required to eject a drop.